1. Field of the Invention
The present invention relates to filtration membranes. More specifically, the present invention relates to polyethersulfone filtration membranes that possess a highly asymmetric pore structure.
2. Background of the Technology
Many types of filtration membranes are available for filtering gases and liquids. Most polymeric membranes are generally made by first preparing a casting solution made up of the chosen polymer in a suitable solvent. The casting dope is then formed into a thin sheet and the polymer is precipitated into a solid phase. Precipitating the polymer into a solid membrane is normally carried out by evaporating the solvent or contacting the polymer with a non-solvent liquid in a quench bath. In many cases, the casting solution also includes a specific concentration of a non-solvent which can affect the porosity of the membrane
Some filtration membranes have a layer of very small pores (termed herein a "skin") on one side, while other membranes do not contain this type of layer (termed herein "skinless"). A skinned membrane is created by quenching a polymeric casting solution of sufficient polymer concentration in a strong non-solvent. The resultant membrane has considerably smaller pores on the "skin" face than on the opposite face.
The casting conditions not only affect whether a skin is produced, but they can also determine the asymmetry of pores within the membrane. For example, a perfectly symmetrical membrane would have pores of the same diameter on both faces and throughout the support structure between the faces. However, a highly asymmetric membrane may have pores that change in diameter by 10,000:1 or more from one face to the other. Asymmetric microfiltration membranes are useful in many applications. For example, such membranes can be used for a variety of filtration applications for purification and testing in the food and beverage industry, water treatment, pharmaceuticals, and in medical laboratories. The membranes are useful in a variety of forms, including, for example, disks and cartridges. Such membranes have become increasingly relevant to the testing industry for uses as diverse as trace metals analysis and medical diagnostics.
Highly asymmetric membrane structures are disclosed in U.S. Pat. Nos. 4,629,563 (Re-examination Certificate No.: B1 4,629,563) and 4,774,039 to Wrasidlo. In these disclosures, the degree of asymmetry from the skin face to the opposite face within the support structure is gradual, rather than abrupt. This allows the membrane support structure to act as a prefilter (or more accurately, as several prefilters of different sizes), and enhances the life and dirt-holding capacity of the membrane by retaining particles that are much larger than the skin pores well before they come into proximity with the skin layer.
The manufacture of the Wrasidlo microfiltration membranes is based on the properties of an unstable dispersion of a membrane casting solution within the binodal or spinodal curves of a phase diagram. With an unstable casting solution, such as that described in Wrasidlo, the membranes are normally produced with constant agitation prior to casting. If the casting solution was not constantly agitated, the polymer-rich phase and polymer-poor phase would separate leading to an undesirable membrane, In addition, during unstable casting, special film exposure conditions are usually required. Furthermore, it is sometimes necessary to provide additional polymers in the casting solution or a heating step to induce the proper phase separation prior to the quenching step.
In addition, polysulfone membrane compositions based on the Wrasidlo patents do not stand up to repeated heating, such as by an autoclave, because they are made of polymers with low glass transition temperatures. Although others have experimented with various polymers in an attempt to overcome some of the disadvantages of prior membranes, they have met with limited success. For example, U.S. Pat. Nos.: 4,933,081 and 4,840,733 to Sasaki ("Sasaki Patents") disclose asymmetric polyethersulfone filtration membranes.
As is known, polyethersulfone can withstand heating and cooling procedures more readily than polysulfone. However, the methods disclosed in the Sasaki patents produce an hourglass-shaped membrane with maximum pore diameters on both surface faces and minimum pore diameters on the inside of the membrane. However, this type of membrane structure is, for many applications, not advantageous because of the internal location of the minimum pore. As disclosed in the Sasaki patents, the position of the minimum diameter pores within the membrane is related to the surface size of the pores. For example, as the layer of minimum pores is positioned deeper and deeper within the membrane, the pore diameters on the surface begin to get larger. For this reason, it is more difficult to manufacture a membrane with a particular external surface pore diameter.
In addition, to provide a 1000:1 asymmetry between the maximum and minimum pore diameters, the internal minimum pores must constrict 1000-fold from the exterior of the membrane to the middle of the membrane. The rapid constriction from the outer surface of the membrane to the inner pore can lead this type of membrane to have lower flow rates and clog more easily than a membrane with a more gradual slope from the largest pore diameter to the smallest pore diameter.
Accordingly, it would be desirable to provide a highly asymmetric polyethersulfone porous membrane having a high degree of water permeability, sufficient strength and rigidity, and that operates efficiently in separations and testing applications, wherein such a membrane could be produced from a more simplified casting process than the prior art.